Question:
Write in an essay format on the given topic with diagrams and needs to include many scientific reference sources and have intermediate level of detail.
Answer:
Introduction:
Earthquakes in New Zealand are predominant because it belongs to the pacific ring of fire. This ring is geologically active. Approximately twenty thousand earthquakes have been recorded each year. Among these strong earthquakes are 200. There are major fault lines. They follow the length of the region. Most of the major faults consist of strike slip faults that are oblique. These faults consist of movements that are vertical as well as sideways
Zones of major active fault line displaying variation in the displacement vector of Pacific Plate. Along the boundary, It is relative to Australian Plate . These fault lines are shown above.
Distribution:
Along the main ranges, most of the earthquakes in this region occur. They run from East Cape innortheast to Fordland in SW (southwest). Then they follow the plate boundary between the Pacific plates and Indo-Australian plate. Along central Alpine Fault, frequency of large earthquakes is less. There is no subduction of the plates. Tectonic forces are accommodated in various manners. The nation's capital, Wellington is the largest city. It is located within this zone that is high risk. Then comes Hastings. After that, Napier comes. Since European settlement, these cities have gone through major earthquakes.
Causes:
The most common cause for the active seismicity in New Zealand is reinjection(Bromley andMajer,2012). Eventhough the New Zealand geothermal projects have been started on a large scale in the late1950's , the reinjection did not commence til 1980's (Sherburn, 1984, Allis et al., 1985). Reinjection is now mandatory at all fields, mainly for environmental reasons It acts as a means of disposing of condensate and brine. Reinjection also provides potential improvement in long-term sustainable utilization of resources by sustaining the pressures through the fluid recharge, particularly where the natural recharge is low. Most fields have at least some potential for induced seismicity related to injection management. The reinjection strategy that are being used
at most of the New Zealand fields has changed during their production history especially in
terms of depths, locations, injection temperature, flow rates pressures and in-situ. The Injection at a relatively shallow depth (500m) was very common as the starting strategy to minimize
rapid reinjection fluid returns to the deeper production aquifers (~1-3km depth). These were
possibly successful only when suitable aquifers could be found for injection and they must be of
intermediate depth and temperature, and isolated by a very low permeability aquitard from production depth aquifers. Most fields now adopts an adaptive injection strategy that depends mostly on deep reinjection, i.e., at or below the depth from which production fluids are been extracted. The Fields where the reinjection depth has changed from shallow to deep include:Kawerau (1992 shallow to 2008 deep), Rotokawa (1997 shallow to 2006 deep), and Mokai (2000 shallow to 2008 deep). At Ohaaki reinjection changed from deep (1988) to shallow (1993). At Ngawha (from 1998) and Ngatamariki (from 2013), injection has thus far been deep.
Fault lines:
Considering North Island, there is a feature concerning large plate boundary. That is known as the North Island Fault System. This system is under stress (constant) because of the plate movement.Australian plates and the Pacific plates are responsible for this movment. Following a continuous line from Wellington coast to the Bay of Plenty southwards, whole system has developed a line of mountain ranges i.e. Rimutakas, Tararuas, Ruahines and Kaweka. There are several major parallel faults In the Wellington area, i.e. Wellington Fault. There are many active faults in the Taupo Volcanic Zone, related with extension as well as rifting in the area. Another series of major parallel faults is there in the South Island. That is known as the Marlborough Fault System They converge further in south. There they produce the Alpine Fault. Significant part of the total plate boundary strain is carried by this fault. Along most of its length this feature is distinct. This is due to Southern Alps. Along its eastern side, they are uplifted. Hence, it is clearly visible from space. In the next 50 years, there is a high risk of a occurrence of a major earthquake. Across the South Island, there are still various relatively minor faults. Here, rupture is less frequent, i.e. those which triggered the Christchurch and Canterbury earthquakes.Geodetic and Structural data related to the Alpine Fault indicates that a major part pertaining the total plate displacement belongs to rapid oblique slip. This slip is on the fault. To the east, over a 200 km wide zone, there is a distribution of the remaining displacement (Norris et al. 1990).